There are some articles about what happened in that time period,
some pictures, and some more information about Haymarket Riot.

After the Knights of Labor was formed, they got into a serious
trouble because of what happened at Haymarket Square. The workers at the McCormick Harvester Company in Chicago went on strike.

Like other companies did, the McCormick Company hired strikebreakers,
so they can replace for striking workers. In May, the workers clashed with strikebreakers outside the factory. Police
were there, and four workers were killed by police.

Other workers were angry about the killings, so they
gathered in Haymarket Square to protest the killings. This was the Haymarket Riot.